It is a common requirement in electronic power substrates to be able to attach devices and be able to dissipate heat developed by the devices in an efficient way. There is a need to optimise the size of the substrate and the electrical/thermal performance of the devices and circuit in varied ambient conditions, where the thermal resistance, balanced against electrical isolation, between the device and the substrate, is higher than desired for optimum performance.
The competing requirements of high dielectric strength (i.e. good electrical insulation), with high thermal conductivity, is satisfied by very few materials. The most suitable materials include diamond, aluminium nitride, and other engineering ceramics, but these are of limited practical opportunity. In practice, a severe compromise is usually made on either the dielectric strength, or on the thermal conductivity. Polymer insulation, for instance, is often used in such applications, but is limited to thermal conductivities of ˜1 Wm−1K−1, and is of course limited to low temperature applications (typically <200° C.).
Conventional anodising (for instance, as disclosed in GB2162694, U.S. Pat. Nos. 4,015,987, 5,687,062) offers reasonable dielectric strength of a couple of hundred volts, but the amorphous oxides formed by conventional anodising offer low thermal conductivity (<1 Wm−1K−1), and suffer from defects on sharp convex radiuses. The defects are partially overcome in WO96/33863, but remain a problem. Furthermore, anodised coatings tend to dehydrate and crack above 100° C. making them unsuitable for high temperature applications. Moreover, on titanium and magnesium, the oxides achieved by anodising are very thin and insubstantial.
Plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), is an enhancement of anodising which overcomes some of these defects. Like anodising, it may be applied to any valve metals (i.e. metals whose oxides present electrically rectifying behaviour), but it is in fact less sensitive to the exact composition of the substrate alloy and can be successfully applied to any common alloys of Al, Mg or Ti.
The resulting oxides do not suffer from the columnar porosity inherent in anodised aluminium, and the consequent defects on sharp convex radiuses and complex geometries. Even on Mg and Ti alloys, thicknesses of several microns of uniform oxide coatings can be achieved. The oxides are well adhered, hard, and physically robust. For instance, their low elastic moduli of just a couple of tens of GPa [“Thermo-physical properties of plasma electrolytic oxide coatings on aluminium”, Curran, J. A. and Clyne, T. W., Surface and Coatings Technology, v.199(2-3), pp.168-176 (2005)] make them strain tolerant, and able to resist thermal cycles of over 500° C. without any damage or chemico-physical change to the coating. They have fine scale porosity (from a few hundred nanometers in diameter down to the limit of detection of most analysis techniques at a few nm).
The oxides tend to be at least partially crystalline, and this has resulted in standard PEO coatings having reported thermal conductivities of ˜2 Wm−1K−1 on aluminium and magnesium [“The thermal conductivity of plasma electrolytic oxide coatings on aluminium and magnesium”, Curran, J. A. and Clyne, T. W., Surface and Coatings Technology, v.199(2-3), pp.177-183 (2005)]. This is higher than the values for the amorphous coatings which result from conventional anodising, but remains an order of magnitude lower than expected values for polycrystalline alumina or magnesia (˜20-40 Wm−1K−1), and is a consequence of the fine grain size (generally measured at tens to hundreds of nanometers) [“The thermal conductivity of plasma electrolytic oxide coatings on aluminium and magnesium”, Curran, J. A. and Clyne, T. W., Surface and Coatings Technology, v.199(2-3), pp.177-183 (2005)] and the significant amorphous phase proportion in the coating. There has been some interest in reducing the thermal conductivities further still, to offer better thermal insulation, and this has been achieved by additions to the electrolyte [“Mullite-rich plasma electrolytic oxide coatings for thermal barrier applications”, Curran, J. A., Kalkanci, H., Magurova, Yu., and Clyne, T. W., Surface & Coatings Technology, v.201, pp. 8683-8687 (2007)].
The application of PEO in power electronics has been described in general terms in WO2006/075176, and the present invention builds on the work outlined in that disclosure. In WO2006/075176, the dielectric strength (breakdown strengths of up to 3.5 kV for coatings of 20-30 μm thickness), and the physical robustness of the coatings, are recognised as being superior to that of anodising. The scope for directly applying electrical circuitry (by such means as electroless copper deposition, thick film deposition, sputtering etc.) to minimise thermally resistive interfaces, is also recognised. These means are favoured by the surface roughness and fine scale porosity which is inherent to PEO coatings.
The state of the art of PEO until now, however, has been limited to upper values of 2-3 Wm−1K−1 in thermal conductivity, and although this has been sufficient for many applications such as high power LED substrates, this has ultimately limited the potential applications and usefulness of this technology as a basis for insulated metal substrates.
Hitherto, insulated metal substrates generally have an upper operating temperature limit of 200-250° C., due to their reliance on polymeric layers for electrical insulation, or due to mismatch between the mechanical properties of the metal substrates and the insulating layers which makes them vulnerable to delamination and coating spallation under the stresses generated by temperature changes. This also severely limits their resistance to thermal shock and thermal cycling.
Known flexible substrates rely on polymer films such as kapton, with low thermal conductivity (˜0.2-0.35 Wm−1K−1 at room temperature), giving low performance in terms of heat extraction.